1. Field of the Invention
The present invention relates to a blow cylinder, which can be used as a component of a blow molding system. More particularly, the invention is a blow cylinder that utilizes a fluid at a minimum pressure to cushion a piston head as the blow cylinder acts to extend a blow needle.
2. Description of the Related Art
A piston assembly in a blow cylinder used to extend a blow needle into a parison typically includes a piston head, a piston rod, and a blow needle. In order to extend the blow needle, air is forced into the head space of the blow cylinder adjacent to the side of the piston head farthest from the blow needle. The air pressure forces the piston head to extend, along with the piston rod and the blow needle, such that the blow needle engages and pierces the parison. A parison is a hollow plastic tube, for example, in the shape of a test tube. After the parison has been pierced, a fluid, e.g., air, can be forced through the blow needle and into the hollow interior of the parison to inflate the parison against the walls of a mold. The features of the mold in conjunction with the inflation pressure mold the container. After inflation, the blow cylinder can retract the blow needle from the molded container.
In a conventional blow cylinder, when the piston head is forced in a direction, the piston head can violently impact a wall that limits the travel of the piston head in the cylinder body. In order to retract the blow needle after inflation, air is forced into the heel space adjacent to the side of the piston head closest to the blow needle. The air pressure forces the piston head to move such that the blow needle disengages the blow molded container. In this step, the side of the piston head farthest from the blow needle can violently impact an end cap of the blow cylinder. The repeated impact of the piston head alternately against a cavity wall and an end cap in the blow cylinder causes wear and metal fatigue in the piston head and in the piston rod, which connects the piston head and the blow needle, resulting in failure of the blow cylinder. The blow cylinder must then be replaced or repaired, generally long before surfaces and components other than the piston head and the piston rod have worn out. The potential for failure of a conventional blow cylinder detrimentally affects the reliability of the entire blow molding system, and the need for frequent blow cylinder replacement or repair increases the cost of operation of the system as a whole.
In order to support the operation of a conventional blow cylinder, the blow molding system must include not only a valve regulating the flow of air to the head space of each blow cylinder for extending the blow needle, but also a second valve regulating the flow of air to the heel space of each blow cylinder for retracting the blow needle. Each of these valves requires its own set of cams or solenoids to control operation of the valve. Even if the two valves can be replaced with a single three-way valve, a three-way valve is more expensive than a valve with only on and off positions.
In general, pneumatic cylinders in which air pressure is used to force the piston head in an extend direction and air pressure is used to force the piston head in a retract direction are referred to as double-acting cylinders. For example, double-acting cylinders are manufactured by Illinois Pneumatics, Inc. of Roscoe, Ill.
Impact of the piston head against a wall or an end cap of a blow cylinder can be cushioned by use of a mechanical spring. A mechanical spring can also serve to effect retract motion of a piston head when the air pressure used to effect extension motion of the piston head is released. For example, a mechanical spring could be located in the heel space between the side of the piston head closest to the blow needle and a cavity wall. However, such a mechanical spring is itself susceptible to breakage resulting from fatigue attributable to repeated cycling. Debris resulting from breakage of the spring can damage other components of a blow cylinder. The spring imposes a set return force for a given location of the piston head with respect to the wall; there is a set return force for a given position of the blow needle. Because the return force at a given position of the piston head cannot be readily adjusted, the blow cylinder cannot be readily optimized for the manufacture of a specific container. Furthermore, over the course of its life, the spring constant of the mechanical spring may change. This can result in the return force for a given position of the blow needle changing over time, requiring adjustment of the blow cylinder, which may be difficult or impossible and requires interruption of manufacture.
In general, pneumatic cylinders in which air pressure is used to force the piston head in a direction and a mechanical spring is used to return the piston head in the opposite direction are referred to as single-acting cylinders with a spring. For example, single-acting cylinders with a spring are manufactured by Illinois Pneumatics, Inc. of Roscoe, Ill.
There thus remains an unmet need for a blow cylinder that is reliable, has long operating life, does not require adjustment over the course of its operating life, and can be readily optimized for use in manufacturing a specific container.